Method to sense temperature in an implantable pump

ABSTRACT

An implantable drug infusion pump for delivering drug therapy is made more reliable and its performance improved by monitoring drug pump temperature. Monitoring pump temperature can also provide for temperature-related drug therapy modification. 
     A pump temperature sensor is read by the infusion pump&#39;s microprocessor. Pump temperature data is stored in pump memory for later access by a remote controller. A simple thermistor or semiconductor temperature sensor can provide fast and reliable temperature monitoring of the pump and/or of a patient by reading the temperature sensor&#39;s value and calculating a temperature therefrom.

This is a continuation of U.S. patent application Ser. No. 11/775,490,filed Jul. 10, 2007, which is a continuation of U.S. patent applicationSer. No. 09/950,154, filed Sep. 10, 2001, both of which are incorporatedherein by reference in their entirety, which is a continuation-in-partapplication of U.S. patent application Ser. No. 09/302,517, filed Apr.30, 1999, to which priority is claimed.

FIELD OF THE INVENTION

This invention relates to implantable drug infusion pumps. Inparticular, this invention relates to a method and apparatus forcontinuously sensing and recording temperature of an implantableinfusion pump.

BACKGROUND OF THE INVENTION

Implanted infusion pumps deliver therapeutic drugs to a patientaccording to a computer program executed by a processor that isprogrammed with drug dosing parameters. Some infusion pumps use amicroprocessor to control a small, positive displacement pump accordingto programming instructions delivered to the microprocessor through anRF programming link so as to permit the implantable pump to be remotelyprogrammed and operated. Other infusion pumps use compressed-gaspropellants instead of a pump to deliver a drug.

Most medical devices, including infusion pumps, are specified to bestored in a particular not-to-be-exceeded temperature range. Storagetemperatures outside the manufacturer's specified storage temperaturerange can damage implantable infusion pumps and for this reason,precautions are normally taken to insure that an implantable infusionpump is not inadvertently subjected to adversely high or lowtemperatures. Monitoring a pump's temperature over time would provide amechanism by which damaging temperature extremes could be identifiedprior to implantation.

In addition, a pump that includes a mechanism by which the pump'stemperature can be monitored might provide drug-delivery performanceimprovements. The flow characteristics of mechanical pumps are oftentemperature sensitive. Temperature compensation of undesirable flowchanges can be achieved using the electrical temperature signal toadjust the flow via the internal controller.

Furthermore, monitoring patient temperature by an infusion pump, eitherremotely, for example at the distal end of a catheter connected to thepump, or at the pump, might allow for drug therapy delivery to bemodified according to the patient's measured temperature, improving theeffectiveness of the therapy.

BRIEF SUMMARY OF THE INVENTION

An implantable drug infusion pump is made more reliable and itsperformance is improved by inclusion of a temperature sensor in thepump, which monitors the pump's temperature. Undesirable temperaturedependencies in an infusion pump's performance can be reduced oreliminated by measuring the pump's actual temperature using a separatetemperature sensor and adjusting the pump's operation accordingly by wayof a computer program designed to modify pump performance according totemperature variations. Drug therapy administered by an infusion pumpcan be automatically or manually adjusted according to the pump's actualtemperature.

In the preferred embodiment, a thermistor, embedded within a pump at anempirically determined optimum location to monitor the overalltemperature of the pump's constituent mechanisms, is operatively coupledto the pump's control microprocessor. The microprocessor's controlprogram is written to read the thermistor's resistance and from thetemperature-dependent resistance of the thermistor, calculate the pump'stemperature.

In at least one alternate embodiment, a temperature sensor external tothe infusion pump can be used to measure a patient's temperature. Suchan embodiment would include using a temperature sensing device, on thedistal end of a catheter for example, providing a faster temperaturesensor and a temperature more closely similar to the core temperature ofa patient.

EEPROM or battery-powered RAM, on-board the microprocessor or in anexternal device, can be used to store the date and time at which amicroprocessor controlling the pump and also monitoring a temperatureprobe, read the pump's temperature. The microprocessor can correlate anelectrically measurable parameter, such as a temperature-dependentresistance of a thermistor for example, to a real temperature. Thepump's temperature history since manufacture and prior to implant into apatient can be stored in memory and subsequently read from memorythereby providing a complete history of the pump's temperature.Historical temperatures stored and read prior to installation might helpinsure that the pump will not fail due to having been frozen or failbecause of exposure to abnormally high temperatures since manufacture,causing a possible electrical or mechanical failure.

Pump temperature data values stored in memory can be read from the pumpprior to installation using a direct-connect-programming link or througha RF programming link, which is commonly used to transfer data to andfrom implantable infusion pumps and described elsewhere in theliterature. See e.g. U.S. Pat. No. 4,676,248, “Circuit for Controlling aReceiver in an Implanted Device” by Berntson.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 discloses a simplified block diagram of an implantable, softwarecontrolled infusion pump that also includes a built-in temperaturesensor.

FIG. 2 shows an alternate embodiment wherein a catheter includes atemperature sensor coupled to a pump, which might include an externalcontroller.

FIG. 3 shows a flow chart illustrating the steps to compensate drugdelivery flow in an infusion pump.

FIG. 4A shows a graph of an uncompensated flow rate of an implantablepump versus temperature.

FIG. 4B shows a graph of a temperature compensation algorithm'sprogrammed relationship between pump revolutions per hour for animplantable pump and temperature.

FIG. 4C shows a graph of a temperature compensated flow rate of animplantable pump.

FIG. 5A shows the temperature profile of an implantable pump over time.

FIG. 5B shows the histogram output of the total time an implantable pumpwas exposed to various temperature ranges.

FIG. 6 shows a flow chart depicting the steps needed to produce ahistogram output of the total time an implantable pump was exposed tovarious temperature ranges.

FIG. 7A shows a graph of an implantable pump's pump cycles verses apatient's body temperature.

FIG. 7B shows a graph of flow rate of an implantable pump versus apatient's body temperature.

FIG. 8 shows a flow chart depicting the steps needed to produce a flowfrom an implantable pump dependent on a patient's body temperature.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a simplified block diagram of the functional elements of animplantable and programmable drug infusion pump 100 having aprogrammable microprocessor 108 and a temperature sensor 150 formonitoring the pump temperature.

The functional elements of the infusion pump 100 are shown in FIG. 1 aresmall, such that the pump can be readily implanted into the abdomen of apatient for purposes of treating chronic diseases, such as diabetes. Animplanted infusion pump might also be used for acute treatment regimens,e.g. to administer chemotherapy drugs or morphine, for example. Areservoir 102 contains a volume of drug to be administered to thepatient by a pump 104, preferably a precision positive displacement pumpcontrolled by the microprocessor 108 and drawing drug material from thereservoir 102.

The pump 104 shown in FIG. 1 is operatively coupled to and responsive toelectrical signals delivered to it from a radio frequency (RF) interfaceunit 106. Electrical signals from the interface unit 106 might, forexample, start and stop the pump 104 and including its delivery rate soas to modulate the delivery of drugs from the reservoir 102 to thepatient. Control circuitry within the microprocessor unit 108 wouldtypically include appropriate electronic drive circuits, the essentialfunction of which is to couple a central processor 108 to the pump 104through appropriate interface circuitry well know to those skilled inthe art. Alternate embodiments of the invention would of course includeimplementing any required pump/CPU interface directly into themicroprocessor, or selecting and/or designing the pump 104 to eliminatethe need for an interface between it and the low power circuits of themicroprocessor. Many commercial grade microprocessors include a plethoraof ancillary circuitry on a single substrate including analog-to-digitalconverters, digital-to-analog converters, counters, timers, clocks andso forth.

The central processor unit 108 controls the amount of drug treatmentadministered to the patient according to program instructions stored ina program memory 110. In the case of a displacement pump mechanism, themicroprocessor might control a drive motor's speed as well as its “on”time.

A temperature sensor 150 is operatively coupled to at least one input ofthe microprocessor 108. The preferred embodiment of the inventioncontemplates that the temperature sensor is a thermistor, the resistanceof which varies with the temperature of the thermistor. Many single-chipmicrocontrollers are fabricated to include an analog-to-digitalconverter which might be employed to measure the resistance of thethermistor by the microcontroller thereby reducing parts count. In usingan on-chip circuit to measure the thermistor's temperature, the controlprogram of the microcontroller can correlate the thermistor's resistanceto a temperature, indirectly measuring temperature by the thermistor'sresistance. Alternate embodiments of the invention would include using atemperature sensor that is a semiconductor for it is well known thatsemiconductor performance characteristics are affected by temperature. Asemiconductor temperature sensor might be fabricated directly on thesame die as the microprocessor.

Still other embodiments of the invention would include a pump thatsenses temperature through a remote temperature probe. FIG. 2 shows analternate embodiment of a temperature-sensing infusion pump 200 whereina temperature sensor 202 is affixed to the distal end of a catheter 204and electrically coupled to a pump or its microprocessor 206 throughappropriate-small gauge wire 208. Such a device might be used to sense apatient's temperature, separate and apart from the pump's temperatureremotely from the pump but still within the patient's body, or inaddition to the pump's temperature for purposes of varying drug dosageaccording to the patient's temperature.

The pump 200 with the catheter 204 connected are implanted in thepatient's body 216 under the skin 214. For remote programming purposes,RF energy 212 flows bidirectionally between the pump 200 and theexternal controller 210 as is commonly done in the art.

Monitoring the pump's temperature over time means that themicroprocessor's 108 control program might periodically scan or read theresistance of the thermistor or other temperature sensing device.Temperature data values read from the temperature sensor might be storedin memory to be read out or analyze at a later time. Alternatively,temperatures that are read and which are outside an acceptabletemperature range limit can be selectively stored reducing the amount ofdata that might need subsequent analysis. In other words, onlytemperatures that are too high or too low might be stored in memory forlater analysis.

Data read from the temperature sensor can be stored in EPROM 114. EEPROM114 is particularly useful in the invention as it readily lends itselfas a repository for long-term data storage regardless of whether or notpower to the memory device has been supplied continuously orinterrupted. Many commercially available microprocessors includeaddressable EEPROM directly on the substrate comprising the CPU furthersimplifying the implementation of a software-limited dosage implantabledrug infusion device. Alternate embodiments of the invention for storingtemperature data include internal RAM memory or would external EEPROM,such as the memory device identified by reference numeral 115.

Historical data of the pump's temperature might be read from the pumpusing the RF programming link 212. Appropriate instruction to themicroprocessor would cause the microprocessor to read and transfer foruploading one or more of the data values stored in EEPROM, RAM or otherdata storage device. A complete record of the pump's temperature fromits manufacture could be re-created providing some assurance that thepump had not been subjected to a damaging temperature extreme.

By use of the invention disclosed herein, the storage temperaturehistory of an implantable infusion pump over time might help identifypumps that are likely to fail after installation. Implantable pumps havea specified storage temperature range over which the implantable pumpcan be stored safely and continue to be suitable for patientimplantation. If the pump is exposed to a higher or lower temperaturethan the storage temperature limit permits, it is possible for the pumpto be damaged and not function as designed. Thus, it is desirable thatthe healthcare provider be aware of historical temperatures that animplantable pump has been exposed to prior to implant. If theimplantable pump has been exposed to temperature damaging extremes, thehealthcare provider can decide not to implant the pump into the patient.

The temperature sensor 150 of FIG. 1 may be used to continuously monitorthe temperature of the implantable pump. Alternatively, a temperaturesensor placed in the packaging of the pump prior to shipment could beused to monitor the temperature of the implantable pump. FIG. 6illustrates a flow chart describing a preferred embodiment of the stepsfor producing a histogram output of the total time an implantable pumpis exposed to various temperature ranges. The histogram is one visualform of output a healthcare provider can quickly examine to determine ifthe implantable pump was exposed to pump damaging temperature extremes.Those skilled in the art will appreciate that other forms of output mayalso be provided and still be considered within the scope of theinvention. For example, the output may be in the form of an audio and/orvisual signal. Such a signal may provide a ready indicator to thepatient's health care provider (such as a red light or a green light) asto whether the pump is suitable for implant. In this embodiment, themicroprocessor within the pump would make the determination based on thedata histogram output.

Referring still to FIG. 6, the temperature 600 of the implantable pumpas sensed by temperature sensor 602 could be read by a temperaturesampler 608 or microprocessor, in the form of an electrical voltage orother electrically measurable quantity such as the resistance of athermistor. The temperature sampler 608 may continuously monitor theimplantable pumps temperature or may be programmed to sample thetemperature at predetermined intervals of time. The predeterminedsampling interval could be determined by the pump manufacturer and varydepending upon the available memory capacity of the pump. Thetemperature sampler signals 609 would be input to a temperaturediscriminator 610 that would separate the signals into three generaltemperature ranges: high temperature 612, normal temperature 614, andlow temperature 616. The temperature ranges could be determined basedupon the specified storage temperature range over which the implantablepump can be stored safely and continue to be suitable for patientimplantation. Those skilled in the art will understand that numeroustemperature ranges could be determined. For example, the temperatureranges could vary in value and number according to the type of drugbeing used or stored in the pump.

The actual temperatures along with the time, date, and rangeclassifications are stored in histogram memory 618 for later retrieval.The time and date could be recorded with a clock operatively coupled tothe temperature sampler 608 or microprocessor. Additionally, thehistogram memory 618 may save the accumulated time the temperature hasbeen in the three predetermined ranges. FIG. 5A, illustrates the type ofinformation that is stored in histogram memory 618. This graph shows thetemperature profile of the implantable pump over time as monitored bythe temperature sensor 602. Prior to pump implant, the healthcareprovider receives the stored accumulated time in each of the threetemperature ranges, for example, via telemetry from the pump.Optionally, the healthcare provider may display the data for reading asshown in FIG. 5B. The histogram format shown in FIG. 5B is one ofseveral possible data display formats that a healthcare provider can useto assist in interpreting the data.

In another embodiment of the invention, the pump reservoir may be filledwith a medicament that is temperature sensitive. For example, themedicament may have a narrow storage temperature range prior to implantof the pump. In this case, the healthcare provider may program the upperand lower temperature limits to a narrower range for the medicamentmonitoring. If the temperature exceeds the acceptable or normal range,the damaged medicament could be replaced as necessary.

As an additional advantage, manufacturing the pump to monitor itstemperature provides another quantum of data that might be useful in thepatient's treatment regimen. After installation into a patient, thetemperature of the pump 100 will quickly adjust to match the temperatureof the body into which it is implanted. Therefore, the pump 100 can alsofunction as a patient temperature probe which tracks patienttemperature. Infused medication dosage might be modulated according to apatient's temperature such that as the microprocessor noticed the pump'stemperature steadily rising the microprocessor might modulate dosagesand/or initiate a communication via the RF link to a health-careprovider. Alternatively, as discussed earlier in FIG. 2, a temperaturesensor could be placed at the distal tip of the infusion catheter withsensor electrical wire(s) running the length of the catheter andinterfacing to the pump via an electrical connector. If the distal tipof the catheter which includes the temperature sensor would be near thebody surface it may detect surface or patient-ambient temperature whichmay not be as therapeutically useful. Therefore, the distal tip of thecatheter should be positioned so it could detect the core temperature ofa patient. The distal tip position may also be determined by the need toprovide a localized infusion of a therapeutic medicament.Advantageously, the sensed temperature of the patient may be used toadaptively administer a drug therapy regimen based on patienttemperature.

FIG. 8 depicts a flow chart illustrating the steps for adaptivelyadministering a drug regimen from an implantable pump based on apatient's body temperature to maximize the therapeutic effect of a drugtherapy. As shown in FIG. 8, the temperature sensor 802 senses apatient's body temperature 801. The temperature signal 804 is read bythe pump cycle controller 806 or microprocessor, in the form of anelectrical voltage or other electrically measurable quantity such as theresistance of a thermistor. The pump cycle controller 806 ormicroprocessor may continuously monitor the patient's body temperatureor may be programmed to sample the patient's body temperature 801 atpredetermined intervals of time. After reading the temperature signal804, the pump cycle controller 806 may apply an algorithm that containsa predetermined proportional relationship between pump cycles and apatient's body temperature as depicted in FIG. 7A. Alternatively, thepump cycle controller 806 may apply an algorithm that contains apredetermined proportional relationship between pump flow rate and apatient's body temperature as depicted in FIG. 7B. Based on thepredetermined relationship, a pump cycle signal 808 is generated anddelivered to the pump mechanism 810 to direct the pump to deliver theproper amount of drug flow 812. The actual patient's body temperaturemay be stored in one or more storage devices for later retrieval by ahealthcare provider using a data link coupled the pump cycle controller806. Additionally, a clock operatively coupled to the pump cyclecontroller 806 may be used to generate a signal representative of thetime and date of the patient's body temperature.

For example, an implanted pump may continuously or at predeterminedintervals sense and store the core or body temperature of a patient.When the sensed temperature increases to a preset value, a low-gradefever is detected which may be therapeutically undesirable. The pump maygradually or abruptly increase the infusion rate, perhaps even providinga bolus infusion, to counteract the low-grade fever. The infusedmedicament could be a fever reducing medicament or perhaps anantibacterial medicament. The low-grade fever may be a consequence oflocalized infection or a systemic reason. When the low-grade fever hasbeen reduced or eliminated, the infusion rate may return to a basal rateor cease. The time dependent temperature record could be sent to thehealthcare provider by telemetry on demand or automatically. This recordwould be useful to monitor the effectiveness of the therapy or to helpthe healthcare provider decide to adjust the infusion rate dependence ona patient's body temperature.

In another embodiment of the invention, an infused drug therapy regimenis adaptively administered according to a temperature compensationalgorithm that adjusts uncompensated flows for temperature so that aconstant flow rate can be achieved. As illustrated in FIG. 3, atemperature sensor 302 would be read by the microprocessor 306 in theform of an electrical voltage or other electrically measurable quantitysuch as the resistance of a thermistor. The temperature signal 304 wouldbe read into the controller 306 where an algorithm would be used todetermine whether the uncompensated flow should be temperaturecompensated. If the controller 306 determines that a correction to theuncompensated flow is needed, then a temperate compensated pump drivesignal 308 is delivered to the pump 310 to produce from the pump aconstant fluid flow rate 312. The temperature compensation algorithm asillustrated in FIG. 4B shows a predetermined relationship between pumpcycles and temperature. As the uncompensated flow rate changes withtemperature, FIG. 4A, the temperature compensation algorithm adjusts theflow rate to provide a constant flow rate as illustrated in FIG. 4C.

An example of a temperature dependent flow rate can be found in thepropellant flow from a propellant pump. Monitoring a pump propellant'stemperature using a temperature probe allows the controller tocompensate drug delivery for the propellant's pressure-temperaturedependence and hence the propellant's temperature. As gaseous propellantchanges temperature, its effectiveness in delivering drug therapy willalso change. Accordingly, by monitoring the propellant's temperature,the microprocessor or other control circuitry can adjust the drugdelivery appropriately to provide for a constant fluid flow delivery ofthe therapeutic.

While the invention has been described with respect to specific examplesincluding presently preferred mode of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variation andpermutations of the above described systems and techniques that fallwithin the spirit an scope of the invention as set forth in the appendedclaims and their equivalents.

1. A method of evaluating an implantable device, comprising: (A)automatically storing in a memory of the implantable device informationcorresponding to sensed temperatures associated with the implantabledevice; and (B) determining whether to implant the implantable devicebased on the information stored in the memory.
 2. The method of claim 1,wherein the storing in step (A) comprises: (i) obtaining from a sensor avalue corresponding to a temperature that the implantable device isexposed to; and (ii) storing the value in the memory.
 3. The method ofclaim 2, wherein the storing in step (ii) comprises: (1) determiningwhether the value is associated with a temperature outside a normaltemperature range; and (2) if the value is associated with a temperatureoutside the normal temperature range, storing the value and a timecorresponding to when the value was obtain in the memory.
 4. The methodof claim 2, wherein the sensor is positioned inside the implantabledevice.
 5. The method of claim 1, wherein the determining in step (B)comprises: (i) retrieving the information from the memory; (ii)determining an amount of time that the implantable device was exposed totemperatures outside the normal range; and (iii) if the amount of timeindicates a reliability of the implantable device was affected by theexposure to temperatures outside the normal range, not implanting theimplantable device.
 6. The method of claim 1, wherein the informationstored relates the period of time that the sensed temperature was notwithin a normal temperature range.
 7. The method of claim 6, wherein theinformation relates to the period of time that the sensed temperaturewas below the normal temperature range.
 8. The method of claim 1,wherein the determining in step (B) comprises: (i) downloading theinformation from the memory; and (ii) rendering the information on adisplay.
 9. The method of claim 8, wherein the rendering only discloseda period of time that the implantable device was exposed to temperaturesoutside the normal temperature range.
 10. The method of claim 1, whereinthe information stored in the memory corresponds to an accumulated timethat the implantable device has been exposed to a range of temperatures.11. A method of evaluating an implantable pump, comprising: (A) storingin a memory a plurality of values corresponding to a temperatureassociated with the implantable pump; (B) retrieving a plurality ofvalues stored in the memory; and (C) determining whether to implant theimplantable pump based on the retrieved plurality of values.
 12. Themethod of claim 11, wherein the storing in step (A) comprises: (i)determining a first value associated with a sensor; (ii) storing thefirst value in memory; and (iii) repeating steps (i)-(ii) after apredetermined period of time.
 13. The method of claim 12, wherein thefirst and second value are selected from the list consisting of avoltage value and a resistance value.
 14. The method of claim 11,wherein the temperature is associated with a temperature sensorpositioned inside the implantable pump.
 15. The method of claim 11,wherein the determining in step (C) is based on whether the implantablepump has been exposed to a temperature outside a safe range oftemperatures associated with a medicament being stored within theimplantable pump.
 16. The method of claim 15, further comprising: (D) ifthe medicament has been exposed to temperatures outside the safe range,replacing the medicament in the implantable pump.
 17. The method ofclaim 11, wherein the retrieved plurality of values provides a historyof temperatures the implantable pump has been exposed to.
 18. The methodof claim 11, wherein the values being stored correspond to temperaturesoutside a normal temperature range.
 19. The method of claim 11, whereinthe retrieving in step (B) is done wirelessly.
 20. A method ofevaluating an implantable device, comprising: (A) determining a periodof time that the implantable device was exposed to a temperature outsidea normal range; and (B) determining whether to implant the implantabledevice based on the determined period of time.
 21. The method of claim20, wherein determining in step (A) comprises: (i) periodicallydetermining a temperature that is associated with the implantabledevice; (ii) if the temperature is outside the normal range, storinginformation in memory corresponding to the temperature and a time thetemperature was sensed; and (iii) using the stored information todetermine the period of time that the implantable device was exposed totemperatures outside the normal temperature range.
 22. The method ofclaim 20, wherein the determining to implant in step (B) is based on acomponent of the implantable device that is susceptible to damage whenexposed to temperature outside the normal range.
 23. The method of claim20, wherein the determining in step (A) is based on sensed temperatureof at least a portion of the implantable device.